Sauté onion in olive oil in a large saucepan until translucent.
Add the Arborio rice and cook for 1-2 minutes.
Gradually add the vegetable broth, stirring constantly until absorbed, then add more.
Stir in asparagus pieces during the last 10 minutes of cooking.
Stir in lemon zest and juice, and Parmesan cheese.
Season with salt and pepper before serving.